Everything you need to know about how positive temperature coefficient resettable fuses work, and how to pick the right one for your application.
TTI has submitted this article. Written by Mohammad Mohiuddin, field applications engineer, Eaton.
Positive Temperature Coefficient (PTC) resettable fuses are simple, cost-effective overcurrent protection devices. These thermistor-based components protect circuits and downstream components against too much current due to an overload or short circuit fault condition. PTCs are compact, self-resetting fuses that provide reliable circuit protection when replacing a one-time fuse is either too costly, impractical or impossible to perform. PTCs are passive devices in that they do not require an external power source to limit current, protecting the circuit downstream.
Resettable PTC fuses are found in most of today’s rechargeable battery systems, protecting both the charging system and its load. Since PTC fuses are small, cost-effective overcurrent protection devices, they are often found in the battery systems for laptops, tablets, mobile phones and wearables. PTCs are almost universally used to protect data lines such as I/O or USB ports.
Other applications for PTCs include automotive and consumer electronics, large appliances, industrial power and transmission equipment, and medical, test, measurement, telecommunications and networking equipment. Resettable PTC fuses can help prevent the additional servicing and downtime that one-time fuses might require. PTCs are also used where replacing fuses is not feasible, such as in sealed laptops, e-book readers, motor drive circuits, and in unreachable places in avionics and aerospace.
In the aforementioned data line protection applications, resettable PTCs are often accompanied by Electrostatic Discharge (ESD) suppressors, which protect against overvoltage.
The basic inner workings of a PTC resettable fuse
The material inside PTC resettable fuses demonstrates a positive temperature coefficient, where the PTC’s resistance increases exponentially with a temperature increase. Under normal operation, PTC resettable fuses insert a trivial amount of resistance with little or no influence on the circuit’s performance. However, an over-current condition heats up the PTC, which causes it to advance to a state of high resistance, often defined as “trips.”
After the source of the overcurrent condition has been removed and the surrounding circuit cools down, the PTC fuse resets itself by reverting to a low state of in-circuit resistance, and normal operation resumes. PTCs can be mounted where they can swiftly sense a temperature increase so a quick response time is certain.
Comparing PTC fuses to one-time fuses
Every fuse has an amperage rating and protects downstream circuit components from damaging overcurrents. Both one-time and PTC resettable fuses connect in series with the load and are not polarity sensitive. PTC fuses reset themselves, whereas one-time fuses open the circuit as a one-time positive disconnect and must be replaced before resuming operation. Both technologies require the removal of the overcurrent condition to resume normal operation. Due to this key characteristic, selection parameters differ—some of which are compared in Table 1, below.
Other key selection parameters for PTC selection
Key parameters are important in determining which PTC resettable fuse is best suited for your application. In addition to Table 1’s parameters, PTC selection key parameters include:
- Initial resistance (Ri): the PTC resettable fuse’s resistance in initial state, measured at +23 °C.
- Post-trip resistance (RTRIP): the maximum resistance measured one hour post-reflow (for surface-mount PTC fuses) or one-hour post-trip (for radial-leaded PTC devices), measured at +23 °C.
- Power dissipation (PD): power dissipated from the PTC fuse when in tripped state at +23 °C.
- Maximum trip time (tTRIP): PTC resettable fuse defined response time from onset of fault current.
Principles of operation for PTC resettable fuses
A polymeric PTC’s response to heat can be plotted as a non-linear curve (see Figure 1 below). Under normal operation, resistance and temperature are in balance at point A, since the amount of heat that’s generated is being successfully dissipated. At point B, an increase in current causes a slight increase in resistance, since most of the excess heat is dissipated. At point C, overcurrent causes heat buildup. The PTC fuse enters a state of high resistance at Point D, after Point C, and limits current flow. (Current flow produces heat at a rate proportional to I2R.) Since PTC resettable fuses are thermally activated, the ambient temperature of the surrounding environment has an impact on its operation.
In a PTC resettable fuse, the relationship between resistance (R) and temperature (T) can be plotted as a non-linear curve. As temperature increases (due to overcurrent), the PTC rapidly increases resistance. The high resistance effectively blocks current flow through the PTC fuse and protects the downstream circuit.
A PTC resettable fuse will trip at or above the trip current (ITRIP) that is listed in the datasheet, up to the Imax current, and protect the circuit. The hold current (IHOLD) is the maximum current a PTC can sustain for a minimum of four hours without tripping (at +23 °C). A PTC trips at or above ITRIP, or the minimum current that will switch or trip a PTC resettable fuse from a low- to high-resistance state (at +23 °C).
However, both IHOLD and ITRIP decrease as ambient temperature increases. In Figure 2 below, the rated hold and trip currents decrease as the ambient air temperature increases. In general, the higher the temperature, the shorter the time it will take to trip. As a rule of thumb, ITRIP is often roughly double IHOLD.
Selecting a PTC resettable fuse requires knowledge about the ambient temperatures in which your application normally operates. A PTC resettable fuse does not trip below or at IHOLD. A PTC fuse isn’t guaranteed to trip until it reaches ITRIP or higher.
PTC resettable fuses are in a state of initial resistance (Ri) when there is little or no power applied. In this cold resistance state (+23 °C), atoms are arranged in a crystalline pattern permitting electrons to easily move around. The more “free” electrons there are to move about, the easier it is for electricity to flow through the PTC, at a state of low resistance.
The materials science on how positive temperature coefficient devices work depends on the particles in the material. Under normal conditions, the current flows easily through the conductive material. But as the current increases, the conductive particles heat up and the internal composition changes, limiting the device’s let-through current. The device remains in this state until the current drops and the material cools down, allowing the material to return to its initial composition.
However, if trip current (ITRIP) is reached, enough heat is generated to rapidly increase the PTC resistance.
At a molecular level, the PTC fuse “trips” as heat causes its atoms to rearrange themselves, locking free electrons down, and causing current to dramatically decrease. With very little current flow, the PTC is effectively in a state of high resistance at or above ITRIP. As the device cools down, electrons are released to flow freely again. At a circuit-level, this resets the fuse and again makes a low-impedance connection so a normal level of current flows again through the protected circuit. In short, the PTC fuse will reset once it cools down, returning to a post-trip resistance value (R1), which is the maximum resistance measured one hour post trip.
Temperature derating and expected ambient operating temperatures
Derating a component refers to choosing one that’s going to be operated at less than its maximum rated parameters. Ambient temperature affects how long it takes to trip a PTC resettable fuse. The higher the temperature, the quicker the trip time. Thus, circuit designers should consider variations in rated current versus temperature and how temperature might affect their application.
Is the application’s circuit in a server room at a well-maintained temperature? Or is it located in a panel on the roof of a building? As an example, note the thermal derating curve from a resettable PTC fuse data sheet as shown in Figure 4 below.
Consider the expected ambient operating temperatures for your application and reduce—or derate—the PTC’s rated current at higher temperatures.
It’s important to know of any possible operating temperature variations the PTC resettable fuse will experience under operation and derate values to ensure proper circuit protection. Ambient temperature conditions can significantly impact the R/T curve seen in Figure 1.
Asking the right questions to get the most out of your PTCs
Circuit protection devices are widely used. Becoming familiar with all aspects of the environment in which the protected circuit operates is crucial.
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